This invention relates to an apparatus for conveying flat goods such as plates, sheets, films and the like, bearing on one side thereof a liquid layer, which apparatus comprises conveyor means moving along an ascending and a descending conveying stretch as well as a transverse, substantially horizontal connecting stretch therebetween, and carrier means for the goods to be conveyed and connected with the conveyor means; the said carrier means holding the goods always in horizontal position while conveying them along the aforesaid stretches.
Such apparatus are used as drying apparatus arranged vertically at several super-imposed levels or similar installations, which increasingly replace the tunnel dryers to economize factory space. When drying still liquid layers, and especially those of low viscosity, it is especially important that these layers are always kept in strictly horizontal position over the entire length of the transportation route, and that they are not exposed to any abrupt changes of velocity or sudden vibrations, as, otherwise, the still liquid mass of the layer can flow off or, at least, the uniformity of the layer can be impaired.
When manufacturing solder-stopping masks on conductor plates (printed circuit boards) a photosensitive lacquer is applied to one face of the plates and is then illuminated and developed (see, for instance, European Patent No. 0002040). In doing so, it is very important that the lacquer coating is as thin as possible and absolutely uniform. Therefore, particular care is also required when converting the liquid layer to a solid coating. This is achieved in the safest manner by leaving the freshly applied layers on the conductor plates exposed to air and completely undisturbed for a certain period of time, e.g., for about 5 to 15 minutes ("pre-airing"). During this time, a portion of the solvent such will evaporate from the lacquer layer that the lacquer will have become sufficiently solid to permit subsequent finishing drying by exposure to stronger air currents in a stove.
In a continuous process such "pre-airing" should, of course, also be carried out continuously. As the "pre-airing" phase of lacquer layers can be relatively long, a vertically arranged pre-airing arrangement is preferred in order to save floor space. For attaining a fully continuous plant operation, the pre-airing station should be equipped with a conveyor apparatus of the initially described type.
The known conveyor apparatus of this type could not fully satisfy the demands made on the qualities of transportation, i.e. absolutely horizontal position and freedom from shock and vibrations; and either their construction or their mechanics are too complicated, cumbersome and/or expensive.
In a known hot air stove for drying printed sheets or the like, marketed by The Argon Service Ltd., Milan, Italy, a solution of the problem of a combined vertical-and-horizontal conveying operation with constant holding of the sheets in horizontal position is attempted by conveying them through two lifts running in opposite directions and by a large number of transfer frames. The lifts comprise each two pairs of endless cycling chains, placed opposite one another, and supporting strips. The transfer frames can be inserted in the lifts and are carried by their supporting strips. In the upper lift region, the transfer frames are pushed by means of a further chain conveyor from the ascending to the descending lift. Loading of the transfer frames takes place by means of a conveyor belt on the feeding side which pushes the printed sheets to be dried onto the next-ascending transfer frame. In order to discharge the sheets, the sheets are withdrawn from the transfer frames when the latter are pushed back in the lower region onto the ascending lift. The dried sheets then drop onto a second conveyor belt for their further transportation.
The above-described known arrangement is mechanically rather involved and can nevertheless not fully satisfy the stiff demands as to uniformity of conveyance such as is required, for instance, in the case of printed circuits, as explained hereinbefore. In particular, the loading and unloading of the transfer frames and their transfer in the upper conveyor region would hardly leave sensitive liquid layers undamaged.
Other apparatus are known, for instance, from the U.S. Pat. Nos. 1,736,866, 3,378,131 and 4,058,908, in which the carriers for the goods to be transported are constituted by rods or bars fastened to conveyor chains to form a kind of grid. The horizontal conveyance from the ascending to the descending conveyor limb is carried out by pushing organs which seize the plate-shaped goods to be conveyed at their margins and push them from the uppermost stage of the ascending limb to the uppermost stage of the descending limb. However, this kind of horizontal transfer is also much too rough for many practical applications, in particular for conveying liquid-covered conductor plates (e.g. printed circuit boards) and suffers, moreover, from the drawback that the plates may be damaged at their margins, where they were seized by the pushing organs, and on their underside, by being pushed over the rods of the afore-mentioned grids.